For example, in general, a plunging type constant velocity universal joint is incorporated into an inboard side (differential side) of a front drive shaft for automobiles, and a fixed type constant velocity universal joint is incorporated into an outboard side (wheel side) thereof. FIG. 23 illustrates an undercut-free type constant velocity universal joint 101 as an example of the fixed type constant velocity universal joint used on the outboard side. Track grooves 103 provided to an outer joint member 102 of the constant velocity universal joint 101 each include a circular-arc portion 103b on an inner-end side of the joint and a straight portion (a linear portion) 103a on an opening side of the joint. Track grooves 105 provided to an inner joint member 104 each include a straight portion 105a on the inner-end side of the joint and a circular-arc portion 105b on the opening side of the joint. A curvature center O1 of a part of a ball-raceway center line x, which corresponds to the circular-arc portion 103b of the track groove 103 of the outer joint member 102, is offset on a joint axial line to the opening side in an axial direction of a joint center Oj. A curvature center O2 of apart of a ball-raceway center line y, which corresponds to the circular-arc portion 105b of the track groove 105 of the inner joint member 104, is offset on the joint axial line to the inner-end side in the axial direction of the joint center Oj. Offset amounts f1 and f2 are equal to each other. Those offsets cause a wedge angle to be formed between the track groove 103 of the outer joint member 102 and the track groove 105 of the inner joint member 104, the track groove 105 facing the track groove 103. As a result, a force of holding balls 106 and a cage 107 within planes obtained by bisection of an operating angle comes into action. Normally, under a state in which the operating angle is 0°, in each of the track grooves 103 and 105, a wedge angle α0 is directed to a direction of opening to an opening side of the outer joint member 102. Normally, the ball 106 is held in contact with the track grooves 103 and 105 at a contact angle (approximately from 30° to 40°), and hence the ball 106 and the track grooves 103 and 105 are held in contact with each other actually at positions on side surface sides of the track grooves 103 and 105, which are slightly spaced apart from groove bottoms of the track grooves 103 and 105. However, for the sake of convenience in illustration, the wedge angle α0 formed between the track grooves 103 and 105 is indicated as that formed between the groove bottoms of the track grooves 103 and 105.
In this structure, each of the balls 106 receives an axial component force in accordance with the wedge angle from the track groove 103 of the outer joint member 102 and the track groove 105 of the inner joint member 104. Thus, the balls 106 press the cage 107 into the opening side of the outer joint member 102. As a result, the axial component force (spherical force) comes into action in spherical fitting parts between the outer joint member 102 and the cage 107 and between the inner joint member 104 and the cage 107. The spherical force leads to heat generation of the constant velocity universal joint, which increases loss of torque-to-be-transmitted.
As a constant velocity universal joint for solving the above-mentioned problem, in a constant velocity universal joint described in JP 3111930 B (Patent Literature 1), track grooves, which form wedge angles opening to opposite sides in the axial direction, are used instead of the above-mentioned plurality of track grooves. This constant velocity universal joint is referred to as counter track type. Description is made of this constant velocity universal joint with reference to FIG. 24.
As illustrated in FIG. 24, a constant velocity universal joint 121 includes an outer joint member 122, an inner joint member 124, balls 126, and a cage 127. A first track groove 123 provided to the outer joint member 122 is formed into a circular-arc shape along an axial direction, and a ball-raceway center line x1 of the first track groove 123 has the curvature center O1. A first track groove 125 provided to the inner joint member 124 is formed into a circular-arc shape along the axial direction, and a ball-raceway center line y1 of the first track groove 125 has the curvature center O2. The curvature center O1 of the ball-raceway center line x1 of the first track groove 123 of the outer joint member 122 is offset to the opening side with respect to the joint center Oj, and the curvature center O2 of the ball-raceway center line y1 of the first track groove 125 of the inner joint member 124 is offset to the inner-end side with respect to the joint center Oj. Those offset amounts are equal to each other. The first track groove 123 of the outer joint member 122 and the first track groove 125 of the inner joint member 124 form a first pair, and the wedge angle α0, which is formed between the first track grooves 123 and 125 under the state in which the operating angle is 0°, opens to the opening side.
Meanwhile, a second track groove 128 provided to the outer joint member 122 is formed into a circular-arc shape along the axial direction, and a ball-raceway center line x2 has the curvature center O2. A second track groove 129 provided to the inner joint member 124 is formed into a circular-arc shape along the axial direction, and a ball-raceway center line y2 has the curvature center O1. The curvature center O2 of the ball-raceway center line x2 of the second track groove 128 of the outer joint member 122 is offset to the inner-end side with respect to the joint center Oj, and the curvature center O1 of the ball-raceway center line y2 of the second track groove 129 of the inner joint member 124 is offset to the opening side with respect to the joint center Oj. Those offset amounts are equal to each other. The second track groove 128 of the outer joint member 122 and the second track groove 129 of the inner joint member 124 form a second pair, and a wedge angle 130, which is formed between the second track grooves 128 and 129 under the state in which the operating angle is 0°, opens to the inner-end side.
The constant velocity universal joint 121 is structured as described above, and hence axial component forces to act on the balls are counterbalanced. As a result, contact pressures at spherical fitting portions can be reduced, which enables reduction of loss of torque-to-be-transmitted. However, in this constant velocity universal joint, the outer joint member 122 includes the second track groove 128 which forms the wedge angle opening to the opposite side in the axial direction, specifically, opening to the inner-end side. Thus, at a high operating angle, the ball drops from the track groove, and hence it is difficult to form high operating angles.
Further, as disclosed in JP 4401745 B (Patent Literature 2), there has also been proposed a constant velocity universal joint in which the track grooves, which form the wedge angles opening to the opposite sides in the axial direction, are used instead of the above-mentioned plurality of track grooves, and in which an end portion on the opening side of each of the track grooves of the outer joint member includes a circular arc having a center on an outside of the outer joint member so that high operating angles are formed. Description is made of this constant velocity universal joint with reference to FIG. 25.
As illustrated in FIG. 25, a constant velocity universal joint 141 includes an outer joint member 142, an inner joint member 144, balls 146, and a cage 147. A first track groove 143 provided to the outer joint member 142 includes a circular-arc portion 143b formed along a spherical portion in the axial direction and a circular-arc portion 143a formed at an end portion on the opening side and curved to an opposite side with respect to the circular-arc portion 143b, the circular-arc portions 143b and 143a being continuous with each other. A region of the ball-raceway center line x1, which corresponds to the circular-arc portion 143b of the first track groove 143, has the curvature center O1, and a region of the ball-raceway center line x1, which corresponds to the circular-arc portion 143a, has a curvature center O3, the curvature center O3 being positioned on an outside in a radial direction of the outer joint member 142. The curvature centers O1 and O3 are each offset to the opening side in the axial direction with respect to the joint center Oj. A first track groove 145 provided to the inner joint member 144 includes a circular-arc portion 145a curved to an opposite side with respect to a circular-arc portion 145b. Although a ball-raceway center line of the first track groove 145 of the inner joint member 144 is not shown, the first track groove 145 of the inner joint member 144 is formed into a shape which is mirror-image symmetrical with the first track groove 143 of the outer joint member 142 with respect to a joint center plane P under the state in which the operation angle is 0°. As illustrated in FIG. 25, the first track groove 143 of the outer joint member 142 and the first track groove 145 of the inner joint member 144 form a first pair, and the wedge angle α0, which is formed between the first track grooves 143 and 145 under the state in which the operating angle is 0°, opens to the opening side.
Meanwhile, a second track groove 148 provided to the outer joint member 142 includes a circular-arc portion 148b formed along a spherical portion in the axial direction and a circular-arc portion 148a formed at the end portion on the opening side and curved to an opposite side with respect to the circular-arc portion 148b, the circular-arc portions 148b and 148a being continuous with each other. A region of the ball-raceway center line x2, which corresponds to the circular-arc portion 148b of the second track groove 148, has the curvature center O2, and a region of the ball-raceway center line x2, which corresponds to the circular-arc portion 148a, has a curvature center O4, the curvature center O4 being positioned on the outside in the radial direction of the outer joint member 142. The curvature center O2 is offset to the inner-end side in the axial direction with respect to the joint center Oj, and the curvature center O4 is offset to the opening side in the axial direction with respect to the joint center Oj. A second track groove 149 provided to the inner joint member 144 includes a circular-arc portion 149a curved to an opposite side with respect to a circular-arc portion 149b. Although a ball-raceway center line of the second track groove 149 of the inner joint member 144 is not shown, the second track groove 149 of the inner joint member 144 is formed into a shape which is mirror-image symmetrical with the second track groove 148 of the outer joint member 142 with respect to the joint center plane P under the state in which the operation angle is 0°. As illustrated in FIG. 25, the second track groove 148 of the outer joint member 142 and the second track groove 149 of the inner joint member 144 form a second pair, and the wedge angle 130, which is formed between the second track grooves 148 and 149 under the state in which the operating angle is 0°, opens to the inner-end side. In this constant velocity universal joint 141, at the end portion on the opening side, the first track groove 143 and the second track groove 148 of the outer joint member 142 include circular arcs respectively having the curvature centers O3 and O4 on the outside in the radial direction of the outer joint member 142. Thus, the balls do not drop off from the track grooves, and high operating angles can be formed.